1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device including a plurality of auxiliary thin film transistors (TFTs) and a method of driving the LCD device.
2. Discussion of the Related Art
With the advance of the information age, devices for displaying information are actively being developed. In particular, flat panel display (FPD) devices having a thin profile, light weight and low power consumption are actively being developed as substitutes for cathode ray tube (CRT) devices. For example, liquid crystal display (LCD) devices, plasma display panels (PDP), field emission display (FED) devices and electroluminescent display (ELD) devices have been researched and developed as FPD devices. Of these FPD devices, liquid crystal display (LCD) devices are widely used as monitors for notebook computers and desktop computers because of their high resolution, high contrast ratio, color rendering capability and superior performance for displaying moving images.
A liquid crystal display (LCD) device relies on the optical anisotropy and polarizing properties of liquid crystal to produce an image. Due to the optical anisotropy of liquid crystal molecules, refraction of light incident onto a liquid crystal depends on the alignment direction of the liquid crystal molecules. Liquid crystal molecules have directional alignment characteristics resulting from their long, thin shapes. The alignment direction of the liquid crystal molecules can be controlled by applying an electric field across the liquid crystal.
FIG. 1 is a schematic cross-sectional view showing a liquid crystal display device according to the related art, and FIG. 2 is a schematic equivalent circuit diagram showing an array substrate for a liquid crystal display device according to the related art. In addition, FIG. 3 is a schematic magnified view of a portion “III” of FIG. 2. FIGS. 1 and 2 show an active matrix liquid crystal display (AM-LCD) device having thin film transistors (TFTs) and pixel electrodes arranged in a matrix form.
As illustrated in FIGS. 1, 2 and 3, an LCD device 10 of the related art includes a first substrate 20 and a second substrates 30 referred to as a color filter substrate and an array substrate, respectively. A common electrode 24 and a pixel electrode 32 are formed on the first substrate 20 and the second substrate 30, respectively, with the common electrode 24 facing the pixel electrode 32. A liquid crystal layer 50 is interposed between the first and second substrates 20 and 30.
A black matrix 26 is formed on the first substrate 20 and a color filter layer 22 is formed on the black matrix 26 and the first substrate 20. The common electrode 24 is formed on the color filter layer 22. The color filter layer 22 may include red, green and blue color filters. The black matrix 26 disposed between adjacent two color filters to block light not passing through a color filter. A plurality of gate lines “G1” to “Gn” and a plurality of data lines “D1” to “Dm” are formed on the second substrate 30 with the gate lines and data lines crossing each other to define pixel regions “P.” A thin film transistor (TFT) “T” is connected to a gate line “G1” to “Gn” and a data line “D1” to “Dm,” and the pixel electrode 32 is connected to the TFT “T.” The TFT “T” and the pixel electrode 32 are formed in each pixel region “P.”
The common electrode 24, the pixel electrode 32 and the liquid crystal layer 50 constitute a liquid crystal capacitor “CLC.” In addition, a storage capacitor “CST” in parallel with the liquid crystal capacitor “CLC” is connected to the TFT “T.” First and second polarizing plates 28 and 34 are formed on outer surfaces of the first and second substrates 20 and 30, respectively.
A gate driver 38 and a data driver 42 are disposed at respective sides of the second substrate. The gate driver 38 is connected to the plurality of gate lines “G1” to “Gn” and sequentially supplies gate pulses to the plurality of gate lines “G1” to “Gn.” The data driver 42 is connected to the plurality of data lines “D1” to “Dm” and supplies data pulses to the plurality of data lines “D1” to “Dm.” The gate pulse is an ON voltage turning on the TFT “T” and a data pulse is a liquid crystal driving voltage for changing the alignment of liquid crystal molecules.
The TFT “T” includes a gate electrode, a source electrode and a drain electrode. The gate electrode and the source electrode are connected to the gate line “G1” to “Gn” and the data line “D1” to “Dm,” respectively. The drain electrode is connected to the liquid crystal capacitor “CLC.” The TFT “T” is turned on and off according to the gate pulse and functions as a switch for application of a data pulse to the liquid crystal capacitor “CLC.”
The LCD device 10 displays images by frames. The gate driver 38 sequentially supplies the gate pulses to the plurality of gate lines “G1” to “Gn” during each frame. In addition, the data driver 42 supplies the data pulses corresponding to the gate pulses to the plurality of data lines “D1” to “Dm.” As shown in FIG. 3, when a gate pulse is supplied to the (n−1)th gate line “Gn−1”, for example, the data pulses are supplied concurrently to all of the plurality of data lines “D1” to “Dm”. Accordingly, the first to mth TFTs “T1” to “Tm” connected to the (n−1)th gate line “Gn−1” are turned on and the data pulses are supplied to the liquid crystal capacitors “CLC” of pixel regions “P” through the plurality of data lines “D1” to “Dm.” As a result, the liquid crystal capacitors “CLC” are charged with a voltage and the alignment of the liquid crystal molecules are changed according to the charged voltage. The change in alignment of the liquid crystal molecules causes a change in transmittance of the liquid crystal layer 50 and the LCD device displays color images by color combination of light transmitted through red, green and blue color filters.
The LCD device 10 further includes a backlight unit 60 under the second substrate 30. Since the LCD device 10 is a non-emissive display device, the backlight unit 60 supplies light to the liquid crystal layer 50 for generating an image. Even though not shown in FIGS. 1 to 3, a seal pattern is formed at a boundary of the first and second substrates 20 and 30 to prevent leakage of the liquid crystal layer 50. In addition, a first orientation film is formed between the common electrode 24 and the liquid crystal layer 50 and a second orientation film is formed between the pixel electrode 32 and the liquid crystal layer 50 to establish an initial orientation of the molecules of the liquid crystal layer 50.
During operation of the LCD device 10, the gate pulse is transmitted from one end to the other end of each of the gate lines “G1” to “Gn.” Since the gate lines “G1” to “Gn” each has a resistance and a capacitance, the shape of the gate pulse is distorted due to an RC delay as the pulse propagates from end to end along a gate line.
FIGS. 4A and 4B are schematic graphs showing the shapes of a gate pulse and a data pulse supplied to first and mth pixel regions, respectively, corresponding to an (n−1)th gate line of FIG. 3. Gate pulses and the data pulses having the shapes shown in FIGS. 4A and 4B are applied to each of the plurality of gate lines “G1” to “Gn” and data lines “D1” to “Dm,” respectively. The first to mth TFTs “T1” to “Tm” are connected to the (n−1)th gate line “Gn−1.” The first and mth TFT “T1” and “Tm” correspond to first and second ends of the (n−1)th gate line “Gn−1,” respectively. FIG. 4A shows an initial shape of an (n−1)th gate pulse “G(N−1)” applied to the first TFT “T1” corresponding to the first end of the (n−1)th gate line “Gn−1” and FIG. 4B shows a final shape of the (n−1)th gate pulse “G(N−1)” applied to the mth TFT “Tm” corresponding the second end of the (n−1)th gate line “Gn−1.”
The (n−1)th data pulse “D(N−1)” is transmitted to the first to mth TFTs “T1” to “Tm” while the gate pulse is applied to the (n−1)th gate line “Gn−1.” In addition, the (n−2)th data pulse “D(n−2)” is transmitted to the first to mth TFTs “T1” to “Tm” while the gate pulse is applied to the (n−2)th gate line “Gn−2,” and the nth data pulse “D(N)” is transmitted to the first to mth TFTs “T1” to “Tm” while the gate pulse is applied to the nth gate line “Gn.” FIG. 4A shows a shape of the (n−1)th data pulse “D(N−1)” transmitted to the first TFT “T1” corresponding to the first end of the (n−1)th gate line “Gn−1” and FIG. 4B shows a shape of the (n−1)th data pulse “D(N−1)” transmitted to the mth TFT “Tm” corresponding the second end of the (n−1)th gate line “Gn−1.”
The (n−1)th gate pulse “G(N−1)” and the (n−1)th data pulse “D(N−1)” each have a rising time and a falling time. A voltage of the (n−1)th gate pulse “G(N−1)” and the (n−1)th data pulse “D(N−1)” increases from an initial value to a final value during the rising time and decreases from the final value to the initial value during the falling time. The voltage of the (n−1)th gate pulse “G(N−1)” and the (n−1)th data pulse “D(N−1)” is maintained at constant value for a time period between the rising time and the falling time. When the (n−1)th gate pulse “G(N−1)” rises to a voltage greater than a threshold voltage “Vth,” the first to mth TFTs “T1” to “Tm” are turned on and the (n−1)th data pulse “D(N−1)” is applied to the liquid crystal capacitor “CLC” to charge up the liquid crystal capacitor “CLC.” When the (n−1)th gate pulse “G(N−1)” falls to a voltage smaller than the threshold voltage “Vth,” the first to mth TFTs “T1” to “Tm” are turned off and the (n−1)th data pulse “D(N−1)” is not applied to the liquid crystal capacitor “CLC.”
As a result, the (n−1)th data pulse “D(N−1)” charges up the liquid crystal capacitor “CLC” in the first pixel region “PXL1” during a first charging time period “Ta(1)” and charges up the liquid crystal capacitor “CLC” in the mth pixel region “PXLm” during an mth charging time period “Ta(m).” Further, the first TFT “T1” is turned off after the (n−1)th gate pulse “G(N−1)” falls during a first off time period “Tb(1)” to have the threshold voltage “Vth” and the mth TFT “Tm” is turned off after the (n−1)th gate pulse “G(N−1)” falls during an mth off time period “Tb(m)” to have the threshold voltage “Vth.”
To prevent a noise signal due to the nth data pulse “D(N),” the (n−1)th data pulse “D(N−1)” is maintained a constant value during a predetermined time period after the (n−1)th gate pulse “G(N−1)” begins to fall, and then begins to fall only after the (n−1)th gate pulse “G(N−1)” voltage falls below the threshold voltage of the first to mth TFTs “T1” to “Tm.” The first to mth TFTs “T1” to “Tm” each are in an ON state even after the (n−1)th gate pulse “G(N−1)” begins to fall until the time when the (n−1)th gate pulse “G(N−1)” reaches the threshold voltage “Vth.” A TFT may be in a slight or partial ON state even when the (n−1)th gate pulse “G(N−1)” has a voltage smaller than the threshold voltage “Vth” due to a property of the TFT device. Were the (n−1)th gate pulse “G(N−1)” and the (n−1)th data pulse “D(N−1)” start to fall simultaneously, the nth data pulse “D(N)” for the nth gate line “Gn” might be applied to the liquid crystal capacitor “CLC” currently charged up with the (n−1)th data pulse “D(N−1)” before the first to mth TFTs “T1” to “Tm” connected to the (n−1)th gate line “Gn−1” are turned off. Accordingly, the nth data pulse “D(N)” may be mixed with the (n−1)th data pulse “D(N−1)” in the liquid crystal capacitor “CLC” causing a noise signal. In order to prevent the noise signal, the (n−1)th data pulse “D(N−1)” is maintained at constant voltage for a predetermined time period after the (n−1)th gate pulse “G(N−1)” begins to fall, and only begins to fall after the (n−1)th gate pulse “G(N−1)” voltage falls below the threshold voltage turning off the first to mth TFTs “T1” to “Tm”.
The initial shape of the (n−1)th gate pulse “G(N−1)” in FIG. 4A is different from the final shape of the (n−1)th gate pulse “G(N−1)” in FIG. 4B due to the equivalent resistance and equivalent capacitance of the (n−1)th gate line “Gn−1.” The (n−1)th gate pulse “G(N−1)” applied to the first TFT “T1” is transmitted to the mth TFT “Tm” through the (n−1)th gate line “Gn−1.” The (n−1)th gate line “Gn−1” includes a conductive material having a resistance and a capacitance. The total resistance and capacitance of the (n−1)th gate line “Gn−1” may be represented by an equivalent resistance and an equivalent capacitance, respectively. The equivalent resistance and the equivalent capacitance of the (n−1)th gate line “Gn−1” generate an RC delay applied to the (n−1)th gate pulse “G(N−1)” transmitted through the (n−1)th gate line “Gn−1.” As a result, the (n−1)th gate pulse “G(N−1)” is distorted such that the rise time and the falling time are extended. As the equivalent resistance and the equivalent capacitance increase the RC delay increases. The distortion of the gate pulse shape due to the RC delay causes a deterioration of the display quality of the LCD device.
As described above, to solve the problem of the interference from the nth data pulse “D(N)” for the nth gate line “Gn,” the (n−1)th data pulse “D(N−1)” is maintained at constant voltage during a predetermined time period after the (n−1)th gate pulse “G(N−1)” begins to fall, and only begins to fall after the (n−1)th gate pulse “G(N−1)” falls to a voltage smaller than the threshold voltage “Vth”
As shown in FIG. 4B, as the falling time is extended due to the RC delay, the mth off time period “Tb(m)” must be extended and the mth charging time period “Ta(m)” is shortened to prevent the noise signal problem due to the nth data pulse “D(N)” for the nth gate line “Gn.” However, when the mth charging time period “Ta(m)” is shortened, the time available for charging the liquid crystal capacitor “CLC” with the (n−1)th data pulse “D(N−1)” is insufficient and the alignment of the liquid crystal molecules is not completely changed to achieve the required transmittance. The insufficient transmittance change results in a non-uniformity of brightness and contrast ratio between right and left portions of the LCD device display, as well as image sticking and flicker. As a result the display quality of the LCD device is reduced.
As a solution for the insufficient charging problem described above, new conductive materials having a relatively low resistance for the gate line have been researched. Additionally, methods using additional circuitry to for gate modulation and employing gate drivers disposed at both ends of the gate lines have been suggested. However, these solutions increase the cost of the LCD device and do not sufficiently address the problems due to the RC delay along the gate line.